1. Field of the Invention
The invention relates to apparatus for electronic watthour metering and, more particularly, to an improved pulse width modulator especially suitable for incorporation into an integrated circuit.
2. Description of the Prior Art
Several methods are known for electronic watthour metering. One method described in U.S. Pat. No. 4,182,983 employs a mutual inductance current transducer to provide a signal which is proportional to the derivative of the current in the circuit being measured. This current derivative signal is supplied as an input to a circuit that produces a pulse width modulated (PWM) signal having a duty cycle that is proportional to current in the metered circuit. The PWM signal is used to control an electronic switch, the input of which is supplied with a signal proportional to voltage on the metered circuit. The output of the switch provides a signal that is proportional to the product of the current and voltage of the metered circuit. The product signal is then supplied to a voltage-to-frequency converter to produce a train of pulses, wherein each pulse represents the consumption of a predetermined quantum of electric energy. The pulses are accumulated to provide a measure of watthours of energy consumed on the metered circuit.
A somewhat simillar technique is employed in apparatus described in U.S. Pat. No. 4,315,212, in which a signal that is proportional to voltage, rather than current, is applied to a PWM circuit having an integrator and a comparator including a feedback loop to produce a PWM signal having a duty cycle proportional to voltage on the metered circuit. It is desirable to provide a PWM circuit with reduced output asymmetry which is amenable to implementation in an integrated circuit configuration.